A flat panel display such as an LCD (liquid crystal display) is currently prevailing as a display device of a personal computer, a television set, etc., in place of a cathode ray tube (CRT) that has hitherto mainly been used. The LCD is a display device that acquires a desired image signal by applying an electric field to a liquid crystal layer having an anisotropic dielectric constant injected between two substrates and by adjusting the strength of the electric field to adjust the amount of light passing through the substrates. A typical type thereof is a TFT LCD using thin film transistors (TFTs) as switching elements.
Since recently the LCD is widely used as a display device of the television set, it needs to display dynamic images. Due to its slow response speed, however, the LCD has hitherto entailed a problem that it may be difficult to display the dynamic images.
To improve such a liquid crystal response speed, a liquid crystal drive (over drive) method is known that applies to a liquid crystal display panel a drive voltage higher than a predetermined gradation voltage for a current frame input image signal, depending on the combination of a one-frame preceding input image signal and the current frame input image signal. Hereinafter, in this description, this drive method is referred to as an overshoot drive.
Although the liquid crystal response speed is known to have an extremely large temperature dependence, some conventional liquid crystal display devices adjust the overshoot drive voltage depending on the use temperature environment. A temperature sensor (thermistor, etc.) for measuring the use temperature is desirably, from its original purpose, disposed within the liquid crystal display panel, but, due to the difficulty arising from reasons of hindering the display, etc., it is attached to another member such as a circuit board.
For this reason, the temperature sensor is placed at a position least influenced by a heat generation action of the other member such as an inverter transformer or a power-supply unit for driving and lighting a backlight light source so that the temperature of the liquid crystal display panel can be detected as accurately as possible. A proper enhanced conversion parameter corresponding to the detected temperature of the liquid crystal display panel is then selected so as to supply proper enhanced conversion data (write gradation data), i.e., an overshoot drive voltage (hereinafter, referred to as OS drive voltage) to the liquid crystal display panel.
Regarding a conventional technique of varying the OS drive voltage depending on the temperature in the liquid crystal display device, Patent Document 1 for example describes one having a temperature sensor that detects a temperature in the device and a disposition form detecting portion that detects a disposition form of the device, so as to allow a proper enhanced conversion data to be acquired all times irrespective of the device disposition form, for the supply to the liquid crystal display panel.
The liquid crystal display device as described above is provided with a gamma correction circuit that performs a gamma correction on input digital image data so as to enable a more natural image display or a display of a quality in accordance with the user's preference. In one example of such a gamma correction circuit, proper conversion data set in accordance with the gamma characteristic of the liquid crystal panel used for example is stored in advance in a lookup table (LUT) set in a ROM, etc. Then, the gamma correction circuit reads out conversion data corresponding to the gradation value of the input digital image data from the LUT to thereby perform the gamma correction.
It is known that the liquid crystal response speed has an extremely large temperature dependence as described above, with the result that the gamma curve varies depending on the change in the ambient temperature. A method is disclosed of variably controlling the gate voltage applied to the liquid crystal panel in accordance with the ambient temperature detected by the temperature sensor (thermistor, etc) so as to correct the temperature-dependent variation (gamma offset) of the gamma curve to keep the gamma curve constant (e.g., see Patent Document 2).